LT1432CS8 datasheet(10/28 Pages) LINER

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LT1432

APPLICATI

I FOR ATIO

1A/DIV

Note that the switch output voltage is nearly identical to

the 12V input during switch on time, a necessary require-

ment for high efficiency, and indicative of an efficient

switch topology. Also note the fast, clean edges on the

switching waveforms, an additional requirement for high

efficiency. The “overlap time” of switch current and volt-

age, which leads to AC switching losses, is only 10ns.

Figure 4 shows the same waveforms when load current

has been reduced to 0.25A, and Figure 5 is at 25mA (note

the scale change for current in Figure 5). The regulator is

now into discontinuous mode as shown by the fact that

switch current has no initial jump, but starts its upward

slope from zero. This implies that the inductor current has

dropped to zero during switch off time, and that is shown

by the “ringing” waveform on the rising edge of switch

voltage. The switch has not yet been turned on, but the

voltage at its output rises and rings as the “input” end of

the inductor tries to settle to the same voltage as its

“output” end (5V).

This ringing is not an oscillation. It is the result of stored

energy in the catch diode capacitance. This energy is

transferred to the inductor as the inductor voltage at-

tempts to rise to 5V. The inductor and diode capacitance

tank circuit continues to ring until the stored energy is

dissipated by losses in the core and parasitic resistances.

The relatively undamped nature in this case is good

because it shows low losses and that translates to high

efficiency. EMI is not increased by operating in this mode.

Figure 6 shows input capacitor current (1A/DIV) with IOUT

= 2A. The theoretical peak-to-peak value (ignoring sloping

waveforms) is equal to output current, and this is indeed

what the top waveform shows. The RMS value is approxi-

mately equal to one half output current. This is a major

consideration because the physical size of a capacitor with

1A ripple current rating may make it the largest component

in the regulator (see output capacitor section). Clever

desigers may hit on the idea of utilizing battery impedance

or remote input capacitors to divert some of the current

away from the actual local capacitor to reduce its size. This

is not too practical as shown by the middle waveform in

Figure 6, which shows input capacitor current when an

additional large capacitor is added about 6" away from the

local capacitor. The wiring inductance and parasitic resis-

tance limit the shunting effect and local capacitor current

is reduced only slightly. the bottom waveform shows input

capacitor current with output current reduced to 0.25A.

Figure 7 shows output capacitor ripple current at loads of

2A, 0.25A, and 25mA respectively starting from the top.

Note that ripple current is independent of load current until

the load drops well into the discontinuous region. The

small steps superimposed on the triangular ripple are

caused by loading of the diode which pumps the power

supply capacitor on the LT1271. Amplitude of the ripple

current is about 0.7Ap-p in this case, or approximately

0.5A/DIV

µs/DIV

Figure 6. Input Capacitor Current

Figure 7. Output Capacitor Ripple Current

µs/DIV

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